Arsenal manager Arsene Wenger says a lack of defensive concentration and bad luck were the two reasons why his side suffered yet another defeat in the Champions League.
The Gunners desperately needed all three points from Tuesday's game but were very poor in the defensive department as they fell to a 3-2 defeat against Olympiakos.
They next face Bayern Munich on October 20 at home before facing the Germans at the Allianz Arena on November 4.
"It is still possible of course to qualify but it is difficult to swallow losing a game like that," Wenger is quoted as saying by the BBC.
"We lost it on a lack of defensive concentration and bad luck. They had four shots on goal and we conceded three goals.
"The turning point at 2-2 is we give them a goal again straight away. If it stays 2-2 for five minutes we win the game.
"A goalkeeper can make a mistake that doesn't explain that we lose the game. It happened to Petr Cech already this season [against West Ham]."